WASHINGTON — Furious last-minute negotiations between the White House and the Senate Republican leadership on Monday secured a tentative agreement to allow tax rates to rise on affluent Americans, but not in time for Congress to meet its Dec. 31 deadline for averting automatic tax increases and spending cuts deemed a threat to the economy.

While the Senate moved toward a vote on legislation to avoid the so-called fiscal cliff, the House was not going to consider any deal until Tuesday afternoon at the earliest, meaning that a combination of tax increases and spending cuts would go into effect as 2013 began. If Congress acts quickly and sends the legislation to President Obama, the economic impact could still be very limited.

Under the agreement, tax rates would jump to 39.6 percent from 35 percent for individual incomes over $400,000 and couples over $450,000, while tax deductions and credits would start phasing out on incomes as low as $250,000, a clear win for President Obama, who campaigned on higher taxes for the wealthy.

“Just last month Republicans in Congress said they would never agree to raise tax rates on the wealthiest Americans,” Mr. Obama said at a hastily arranged news briefing, with middle-income onlookers cheering behind him. “Obviously, the agreement that’s currently being discussed would raise those rates and raise them permanently.”

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President Obama discussed the tentative agreement Monday at the White House, with middle-income taxpayers behind him.Credit
Luke Sharrett for The New York Times

Democrats also secured a full year’s extension of unemployment insurance without strings attached and without offsetting spending cuts, a $30 billion cost.

As negotiators tied up the last points of dispute, officials said that the two top Democrats on Capitol Hill — Senator Harry Reid of Nevada and Representative Nancy Pelosi of California — had signed off on the agreement. In an effort to win over other Democrats uneasy with the proposal, Vice President Joseph R. Biden Jr., who had bargained directly with Republican leaders, traveled to the Capitol on Monday night for a 90-minute meeting with his former Senate colleagues.

“I feel very, very good,” Mr. Biden said after the meeting. “I think we’ll get a very good vote.”

In one final piece of the puzzle, negotiators agreed to put off $110 billion in across-the-board cuts to military and domestic programs for two months while broader deficit reduction talks continue. Those cuts begin to go into force on Wednesday, and that deadline, too, might be missed before Congress approves the legislation.

To secure votes, Mr. Reid also told Democrats the legislation would cancel a pending congressional pay raise — putting opponents in the politically difficult position of supporting a raise — and extend an expiring dairy policy that would have seen the price of milk double in some parts of the country.

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Senator Mitch McConnell, who negotiated with Vice President Joseph R. Biden Jr., returning to his office on Monday.Credit
T.J. Kirkpatrick for The New York Times

Anticipating Senate approval of the deal, Speaker John A. Boehner late Monday said the House would “honor its commitment to consider the Senate agreement if it is passed. Decisions about whether the House will seek to accept or promptly amend the measure will not be made until House members — and the American people — have been able to review the legislation.”

The nature of the deal ensured that the running war between the White House and Congressional Republicans on spending and taxes would continue at least until the spring. Treasury Secretary Timothy F. Geithner formally notified Congress that the government reached its statutory borrowing limit on New Year’s Eve. Through some creative accounting tricks, the Treasury Department can put off action for perhaps two months, but Congress must act to keep the government from defaulting just when the “pause” on pending cuts is up. Then in late March, a law financing the government expires.

And the new deal does nothing to address the big issues that Mr. Obama and Mr. Boehner hoped to deal with in their failed “grand bargain” talks two weeks ago: booming entitlement spending and a tax code so complex that few defend it anymore.

Though the tentative deal had a chance of success if put to a vote, it landed with a thud on Capitol Hill. Republicans accused the White House of “moving the goal posts” by demanding still more tax increases to help shut off across-the-board spending cuts beyond the two-month pause. Democrats were incredulous that the president had ultimately agreed to around $600 billion in new tax revenue over 10 years when even Mr. Boehner had promised $800 billion. But the White House said it had also won concessions on unemployment insurance and the inheritance tax among other wins.

Still, Democrats openly worried that if Mr. Obama could not drive a harder bargain when he holds most of the cards, he will give up still more Democratic priorities in the coming weeks, when hard deadlines will raise the prospects of a government default first, then a government shutdown. In both instances, conservative Republicans are more willing to breach the deadlines than in this case, when conservatives cringed at the prospects of huge tax increases.

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Senator Harry Reid, the Democratic leader, may seek a vote on a stopgap measure on tax cuts backed by President Obama.Credit
J. Scott Applewhite/Associated Press

But as night fell on New Year’s Eve, senators seemed worn down and resigned. “Everybody by this time is angry, but sooner or later this has to be resolved,” said Senator Orrin G. Hatch of Utah, the ranking Republican on the Finance Committee.

Even House Republicans signaled that enough of them, in combination with Democrats, could most likely pass the legislation, just weeks after Republicans shot down Mr. Boehner’s proposal to raise taxes only on incomes over $1 million.

“I don’t want to say where I am until I read the legislation, but it is certainly better than the alternative,” said Representative Charlie Dent, Republican of Pennsylvania.

With a deal agonizingly close, official Washington still prepared for the worst. The Defense Department prepared to notify all 800,000 of its civilian employees that some of them could be forced into unpaid leave without a deal on military cuts. The Internal Revenue Service issued guidance to employers to increase withholding from paychecks beginning Tuesday to match new tax rates at every income level.

“No deal is the worse deal,” said Senator Joseph I. Lieberman, independent of Connecticut, rejecting the assertions of liberal colleagues that no deal would be better than what they would see as a bad deal.

However, new wrenches were gumming up the machinery. Democrats put out late word that Republicans wanted the threshold at which inherited estates would be taxed to be indexed to inflation, a nonstarter for them. Republicans said they needed to see what cuts would pay for the $24 billion needed to put off across-the-board spending cuts.

But with Republicans and Democrats grumbling, it was clear that a deal hashed out through intense talks between Mr. Biden and Senator Mitch McConnell of Kentucky, the Republican leader, had given both sides provisions to cheer and to jeer.

Under the deal, tax rates on dividends and capital gains would also rise, to 20 percent from 15 percent, on income over $400,000 for single people and $450,000 for couples. The deal would reinstate provisions to tax law, ended by the Bush tax cuts of 2001, that phase out personal exemptions and deductions for the affluent. Those phaseouts, under the agreement, would begin at $250,000 for single people and $300,000 for couples.

The estate tax would also rise, but considerably less than Democrats had wanted. The value of estates over $5 million would be taxed at 40 percent, up from 35 percent. Democrats had wanted a 45 percent rate on inheritances over $3.5 million.

Under the deal, the new rates on income, investment and inheritances would be permanent, as would a provision to stop the alternative minimum tax from hitting middle-class families.

Jennifer Steinhauer and Robert Pear contributed reporting.

A version of this article appears in print on January 1, 2013, on Page A1 of the New York edition with the headline: Tentative Accord Reached To Raise Taxes On Wealthy. Order Reprints|Today's Paper|Subscribe